refactor(lang): rename the fn keyword to function

Expand the function-declaration keyword to the full word across the whole
language and toolchain:
  fn name(...) -> T { ... }   ->   function name(...) -> T { ... }

Done as a self-hosting migration: teach the parser both spellings, reseed,
rewrite every .ludic definition to `function`, then drop `fn`. The compiler
now rejects `fn`. Touches the parser, all selfhost/tools/runtime/example/test
sources, the grammars (TextMate shared+vscode, ludic_syntax.h, JetBrains
LudicTokens.kt), the LSP and formatter, the Python doc/vocab tools
(check-impl, check-docs, validate, palette, test-lsp), and the docs
(fences, prose, kw-fn -> kw-function).

Reseeded; C-free bootstrap fixpoint holds. All suites green (45 regression,
24 self-host, 29 tool); the docs site generates and check.py passes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/x/main.ludic

@@ -18,7 +18,7 @@ program X {
   import "selfhost_test.ludic"
   import "test.ludic"
 
-  fn usage() -> void {
+  function usage() -> void {
     print("x — the Ludic task runner (run from the repository root)")
     print("")
     print("build & run:")
@@ -43,7 +43,7 @@ program X {
   }
 
   # a positional argument, or a default when absent
-  fn argn(i: int, dflt: ptr) -> ptr {
+  function argn(i: int, dflt: ptr) -> ptr {
     if (i < arg_count()) { return arg(i) }
     return dflt
   }